Re: Enlighten me about spark plugs

[jim-moffatt]

I’ll try to explain what happens if you use plugs that are too hot. I have experienced this on two motors.
At full speed the plugs get to hot and the mixture pre-ignites resulting in a drop of rpm to a much lower speed. Then
since there is less heat the plug cools off and the motor starts running faster. On a 3.5 Chrysler air cooled motor it cycled over and over.
On a Johnson A-50 the cycles were less clear but it did not run well.

On the Chrysler I had inadvertently installed a H10C instead of the correct H8C.
On the A-50 I was simply experimenting. I tried D9s, D14s, D16s and D21s.
Ended up using D16s.